The present application relates to a method and an apparatus for modifying a raster-scanned video signal such that the modified video signal will still be viewable on a display device, such as a television screen, but that following recording of the modified video signal, playback of the recorded modified signal will be adversely affected.
The present application provides a way of preventing unauthorised copying of an original video signal, and may be used, in particular, to deter copying onto tape of Pay-Per-View video signals, and video-to-video (tape-to-tape) copying.
Video piracy is a significant problem for broadcasters and distributors of video signals, since the unauthorised copying and distribution of video recordings by pirates can impact drastically on the revenue generated by a broadcaster or distributor through legitimate sales. It is therefore desirable to prevent video pirates from making unauthorised copies of video signals.
This is best achieved by modifying the original video signal such that when it is recorded the recorded signal cannot be satisfactorily played back.
In known protection techniques, the unauthorised recording of the video signal is made less enjoyable to watch by the interaction of the original signal to which the protection has been applied with the electronic components in either the video cassette recorder or the television receiver itself. For example, making an unauthorised copy of the video signal too dark to be viewed satisfactorily may be achieved by adding to the original video signal a pulse which is significantly larger than that part of the signal which carries the picture information. The position at which the pulse is added depends on the way in which the circuits in the television receiver or video recorder process the signal. When the modified signal is processed by the automatic gain control circuits of a video cassette recorder, the amplitude of the signal is perceived as being that of the added pulse and not that of the portion of the signal carrying the useful information. Consequently, the video cassette recorder or television receiver amplifies the received signal by a smaller factor than if the pulse was not present. As a result of this the information-carrying portion of the signal is not therefore amplified enough to be seen satisfactorily when reproduced.
Such methods have however a number of drawbacks. Methods which rely on the automatic gain control of the video cassette recorder, such as adding a large pulse to the signal, tend to result in a modified signal that cannot itself be viewed on a television through the video channel regardless of whether the signal is being or has been recorded.
An alternative protection technique described in International Patent Application WO 01/76240 involves the removal of a small number of horizontal synchronisation pulses from the blanking section of the signal, so that an unauthorised recording of the signal cannot be properly synchronised by the TV receiver on which it is to be played back. As a result, the resulting picture playback can be poor.
A further technique, disclosed in International Patent Application WO 96/31878, relies on inserting a pulse into the colour burst information portion of the signal section of the signal, such that automatic gain control circuits that rely on the average dc level of the colour burst to determine the necessary amplification of the signal, make such amplification too small. An opposing pulse signal having a magnitude sufficient to offset the change in dc level of the colour burst portion caused by the pulse signal, and optionally a second pulse, are inserted somewhere from the last half of the remainder of the back porch of the signal to the end of the start of the picture information portion.
We have found that this technique can be unreliable in practice, and furthermore has the disadvantage that by inserting a pulse into the colour burst part of the signal, the resulting picture quality is detrimentally affected.
Both of these techniques rely on the components employed in the video cassette recorder or the television receiver, and, in some cases, certain video recorders or television receivers may have an arrangement of components that is not susceptible to the adverse picture effects caused by the modified signal. Thus, the modified video signal can still be played without significant detriment to the picture playback and the anti-copy protection applied to the modified signal is rendered useless.